Pyrethroid resistance in Culex quinquefasciatus from West Africa

Laboratoire de Lutte contre les Insectes Nuisibles, ORSTOM, Montpellier, France, *OCCGE, Institut Pierre Richet, BP 1500, Bouaké 01, Côte d'Ivoire, †INRA, Laboratoire de Biologie des Invertébrés, 123 Bd Meilland, 06606 Antibes, France, and ‡CNRS UMR 5554, Laboratoire de Génétique et Environnement, Institut des Sciences de l'Environnement, Université de Montpellier II (CC 065), 34095 Montpellier, France Abstract .Pyrethroid resistance was investigated in thirty‐three samples of Culex quinquefasciatus Say from twenty‐five cities in Côte d'Ivoire and Burkina Faso. Permethrin resistance ratios at LC 50 ranged from 9.5‐ to 82‐fold in Côte d'Ivoire and from 17‐ to 49‐fold in Burkina Faso. For deltamethrin, resistance ratios were lower and ranged from nine to thirty‐eight in both countries. A strain was selected with permethrin to investigate resistance mechanisms. After forty‐two generations of selection, permethrin resistance level reached 3750‐fold, but deltamethrin resistance remained unexpectedly unchanged. This indicated that a specific mechanism was involved in permethrin resistance. Synergist assays and biochemical tests indicated that resistance was partly due to P450‐dependent oxidases. A target site insensitivity ( kdr ) was also involved, associated with DDT cross resistance and a dramatic loss of permethrin knockdown effect on adults. This resistance should be taken into consideration when planning the use of pyrethroid‐impregnated materials in urban areas, as Culex is by far the main source of nuisance. Any failure in nuisance control due to resistance is likely to demotivate people in using impregnated materials.